Device for heating of liquids

ABSTRACT

A solar collector has a first plate for collecting solar energy, a second plate wherein there is an intermediate space between the two plates, and a quantity of liquid which is present between the plates for absorbing the energy collected by the first plate. The solar collector has a framework for fixing the plates at a mutual distance along the edges thereof so as to seal the space between the plates and to create space between the plates which comprises the liquid.

[0001] With the increasing environmental awareness alternatives to the present heating of domestic and tap water are being sought increasingly more often. If gas and/or electricity consumption can be reduced, there will be less need to utilize fossil fuels. This results in a lower emission of harmful substances. Solar collectors are required to convert solar energy into heat. The solar energy can be stored in many ways. A much applied method of storing the solar energy is by means of a solar boiler. A solar boiler is a tank in which water is stored which has been heated by the sun. The energy stored in the water can be transferred to for instance tap water by means of heat exchangers.

[0002] The so-called solar collectors operate with liquids as medium. In the existing solar collectors however, only a relatively small part of the energy radiated by the sun per surface area is absorbed. The efficiency is therefore low. The object of the present invention is to realize a solar collector with a high efficiency, i.e. one which converts a greater percentage of the energy received per surface area into energy absorbed by the medium.

[0003] The present invention provides a solar collector, comprising:

[0004] a first plate for collecting solar energy,

[0005] a second plate wherein there is an intermediate space between the two plates, and

[0006] a quantity of liquid which is present between the plates for absorbing the energy collected by the first plate.

[0007] This device has the advantage that the medium is in direct contact with the whole surface area of the plate which comes into contact with the solar energy in the first instance. This is a great improvement relative to existing solutions because in existing solutions the medium flows through a pipe which is arranged against an energy-absorbing plate, wherein the greater part of the energy has to travel a longer distance in order to reach the medium and there is a much smaller surface area between the medium and the metal through which the medium flows in order to transfer the heat. These two factors result in a lower energy absorption and greater losses.

[0008] In the present invention there are two possibilities for the solar energy to be absorbed by the medium. The first is direct irradiation into the medium and the second is absorption by the first plate, whereafter this is transferred to the medium. For this purpose a first embodiment of the invention comprises a framework for fixing the plates at a mutual distance along the edges thereof so as to seal the space between the plates and to create space between the plates which enclose the liquid.

[0009] A second preferred embodiment provides ribs placed between the first and second plate in a manner such that, as seen from bottom to top, they substantially seal the space between the two plates, so that a zigzag-shaped throughflow channel for the liquid is created inside a space enclosed by the plates and the framework. Such a construction enables the liquid to flow through a narrower but longer channel through the absorption means. If a throughflow channel becomes too wide, the flow speed in the absorption means will not be the same everywhere, or will at least have a greater variation than in the case of narrower channels, whereby the heat-transfer efficiency will deteriorate. The slowly flowing parts of the medium will be situated in the absorption means for longer and thereby absorb less heat, since heat absorption depends on the temperature difference.

[0010] A further preferred embodiment of the invention provides that the ribs are arranged substantially parallel on two parts of the framework placed opposite one another. This embodiment is brought about in practice by liquid channels which run perpendicularly in zigzag shape through the heat absorption means or a plurality of channels which run parallel to each other through the absorption means. In the first case it is possible to suffice with one inlet and one outlet per collector or part thereof within a framework, and in the second variant it appears necessary to arrange one inlet and one outlet for each channel, which inlet and outlet means will be further described later.

[0011] A further preferred embodiment of the present invention provides a solar collector wherein the first and the second plate are welded together along the edges with edge joints. This embodiment is preferably further provided with welded joints parallel to two parallel edges which extend from a side wall to a predetermined point situated at a distance from the opposite side of the collector as seen from the start of the welded joint. In this embodiment the plates are preferably also provided with weld points in a predetermined pattern for holding the plates together. The plates preferably move apart between the edge joints, welded joints and weld points. If two plates of stainless steel are welded together along the edges and subsequently provided with the welded joints and the weld points, it then becomes possible to cause the plates to move apart between the edge joints, welded joints and weld points. This can take place by pumping a liquid under gradually rising high pressure between the plates. These plates will then move apart between the welds. Bulges are hereby created, and thereby a space between the plates. This space can be used for throughfeed of the quantity of liquid which is used to absorb the energy collected by the first plate. This embodiment has the particular advantage that it can be manufactured inexpensively.

[0012] A further preferred embodiment of the present invention provides inlet and outlet means to allow liquids to flow in and out of the solar collector. In this case the medium can be supplied in cold state and be discharged in hot state after heat absorption. The throughflow speed can be adjusted. Depending on the quantity of energy which can be absorbed per unit of time and the throughflow speed, it is possible to regulate how great the difference in temperature Δt must become.

[0013] A subsequent preferred embodiment of the present invention has the provision that the first plate consists of a non-transparent material with high heat-conducting properties. In this embodiment all solar energy is first collected by the plate and transferred to the medium. The medium does not therefore receive any direct solar energy. This is nevertheless efficient, for instance in the light of the durability of the installation.

[0014] In a further preferred embodiment of the present invention this first plate is provided with means which improve the absorbing properties of the plate. This can be a coating, and this coating can be provided with black colouring agents. An object of such a coating is to increase the absorption coefficient of the absorption surface.

[0015] Another object of the coating is to reduce the emission coefficient of the absorption surface. Owing to good absorbing properties and a high coefficient of heat conduction, the heat is absorbed efficiently by the first plate and fed through to the medium, thereby increasing the efficiency of the collector. An example of the choice of material for the first plate is stainless steel, since stainless steel is a cost-effective, inexpensive material. The direct contact of the liquid with the heat-absorbing plate enables the production of solar collectors from such inexpensive materials instead of from expensive materials with a high heat conduction such as copper. The possibility of applying economical materials such as stainless steel, while highly efficient collectors are still possible, makes advantageous application of collectors viable.

[0016] A further preferred embodiment of the present invention has the provision that a cover plate is mounted above the first plate for limiting heat losses, for instance due to convection. Since the first plate becomes hot under the influence of the solar energy, there will be a higher temperature than the medium, no matter how efficient the heat transfer to the medium may be. This temperature difference is necessary, since without temperature difference there is no heat transfer. This higher temperature is probably also higher than the ambient temperature. This temperature higher than the ambient temperature will ensure that a part of the absorbed heat is lost. In order to limit these losses, it is efficient to arrange at some distance from the first plate a cover plate which limits these losses. This can be done in two ways. The first way is to prevent radiation energy from the first plate by means of reflection. The second way is to stop heat flow by means of air which is heated at the surface of the first plate. Retention of this hot air increases the temperature of the air between the first plate and the cover plate and thereby prevents new air repeatedly being heated on the first plate.

[0017] Since the cover plate can also block solar energy, a further preferred embodiment of this invention is that the cover plate is provided with a reflection-reducing surface. A treatment for preventing energy losses on the underside of the cover plate is also provided.

[0018] A further embodiment of the present invention has the provision that the solar collector and the cover plate are contained in a housing. This housing serves on the one hand to enable positioning of the components relative to each other and on the other to insulate these components and the space between these components. These insulating means are then situated on the inside of the housing and serve to reduce heat flows on the sides and underside of the device.

[0019] A further preferred embodiment of the present invention provides a second plate with the lowest possible coefficient of heat conduction. This has the advantage that the heat losses to the rear side of the solar collector are reduced.

[0020] A further preferred embodiment of the present invention has the provision that the invention is disposed in a position wherein the average angle relative to the solar radiation is as perpendicular as possible. The reflection of the solar radiation by the cover plate and the first plate will then be as low as possible. This increases the efficiency of the device.

[0021] A further preferred embodiment of the present invention provides a rotatably disposed device so that it is positioned as perpendicularly as possible relative to the solar radiation. This method comprises the following steps of:

[0022] determining the direction of the sun,

[0023] calculating the hereby desired position of the device,

[0024] displacing the device so that the desired position is assumed. This application ensures that the device has the highest possible efficiency during the whole time that the sun is “on tap”.

[0025] Further advantages, features and details of the present invention will become apparent upon reading of the following description of a preferred embodiment with reference to the associated figures, wherein:

[0026]FIG. 1 is an exploded view of an embodiment of the present invention in perspective, wherein the upper plate is shown as transparent;

[0027]FIG. 2 is an exploded view of another embodiment of the invention;

[0028]FIGS. 3a and 3 b show respective partly cut-away views of details III of FIG. 1 and FIG. 2;

[0029]FIG. 4 is a cross-section of a part of the collector;

[0030]FIG. 5 is a cross-section of the collector in a housing;

[0031]FIG. 6 shows a graph of the heat absorption of a collector over a period of a number of hours.

[0032]FIG. 1 shows the first plate 2 which is drawn at a distance above the rest of the embodiment. Also shown is the framework consisting of parts 4, 5, 6 and 7 which together form a rectangle. Parts 4 and 5 respectively 6 and 7 are herein situated parallel to each other. Relative to first plate 2 the second plate 3 is situated on the other side of the framework consisting of parts 4, 5, 6 and 7. The first plate, parts 4, 5, 6 and 7 and the second plate together form a basic embodiment of the present invention.

[0033] Liquid openings 13 and 14 serve as inlet and outlet for the medium, for instance water or glycol or any other liquid. This medium flows through the heat-absorbing means, also referred to as collector. In order to have this flow through the collector proceed more efficiently than from the one side to the other over the whole width, there are provided ribs 8, 9, 10, 11 and 12. These together form on the one hand a longer path for the liquid through the collector and on the other a narrower path through the collector than if these ribs were not present. The advantages hereof have already been described above. The longer path thus implies that liquid flows from 13 via the channel “between 4 and 8”, the channel between “8 and 9”, the channel between “9 and 10”, the channel between “10 and 11”, the channel between “11 and 12” and the channel between “12 and 15 to outlet 14”. Another arrangement of the ribs could be such that parallel single channels are created which are then each given an own inlet and outlet hole. The advantages are herein substantially the same as in the embodiment with a zigzag channel.

[0034] The embodiment in FIG. 2 substantially differs from the embodiment of FIG. 1 in that the inlet holes and outlet holes 14 and 16 are not arranged in the second plate, but in the side parts 4 and 5 of the framework. This has structural advantages when it is difficult to arrange conduits on the rear side of the collector. This may be the case for instance if the device as a whole must be rather flat.

[0035] The enlargements of FIG. 3 elucidate the differences between inlet and outlet holes on the sides of the collector and on the underside of the collector. It can be seen clearly that inlet holes on the side enable a much flatter construction in the case of placement on a roof. If a flat construction is not necessary or it is practical to lead the pipes directly through the roof, the choice for inlet and outlet holes on the underside is more advantageous.

[0036]FIG. 4 is a cross-section of a part of collector 1. Shown are the first plate 2 provided with a coating 2 a and second plate 3 with which the hollow space H is bound. The liquid serving as medium in the heat transfer runs through this hollow space H.

[0037] In FIG. 5 is shown a collector which is contained in a housing. The collector is shown here by first plate 2 and second plate 3 and hollow space H. The collector is enclosed close-fittingly by side walls 25 and 26 of the housing. Situated directly below and above the collector are air compartments in L1 and L2. Under L1 is situated an insulation layer 21 which is in turn placed on bottom plate 24 of the housing. Above L2 the housing is sealed by cover plate 23. This cover plate 23 is intended for the purpose of reducing heat losses from the collector. Heat losses from a collector can take place in two ways, viz. by radiation and convection. The coating on the first plate preferably has a very low energy coefficient, i.e. the radiation losses are negligible. The losses due to convection are however not negligible. The cover plate thus ensures that energy which would be lost through convection is to a great extent retained. A cover plate reflects solar radiation. However, with a correct coating radiation losses through reflection are relatively small compared to the gain achieved by preventing energy losses from the collector due to convection.

[0038] A heated collector will also lose energy on the underside. In order to limit these energy losses a layer 21 of insulating material is provided. Insulation can optionally also be arranged (not shown) on the sides between the collector and walls 25 and 26.

[0039] The thickness of air layers L1 and L2 influences the magnitude of the energy losses. The thickness of insulating layer 21 likewise affects the magnitude of the energy losses.

[0040] The graph (FIG. 6) shows the heat production of a typical collector according to the present invention wherein the collector is manufactured from a metal. In practice the choice of material has little influence on the yield, i.e. in the order of magnitude of several percent. The application of a cover plate is of greater importance, in the order of magnitude of 10-15%.

[0041] A further embodiment 70 (FIG. 7) consists of two stainless steel plates 71, 72. These two plates are welded together along edges 73, 74, 75, 76 by means of laser welding, wherein a liquid supply 81 is arranged in a recess 80. Welded joints 71, 78, 79 are then arranged, which designate the boundaries of a liquid throughflow channel. Weld points 82 are subsequently arranged. These weld points serve to hold plates 71, 72 together. In this embodiment the welding operations take place by means of a laser-welding process.

[0042] In order to enable the passage of liquid between the two plates the plates are pressed apart by supplying a liquid through opening 81 under high pressure. This creates (FIG. 8) a hollow space between the two plates, whereby the liquid (the heat-absorbing medium) can flow through the solar collector. For this purpose the collector can further be provided with liquid inlet and outlet holes 83, 84.

[0043] These and other embodiments can for instance be embodied in sheet steel with a thickness from 0.3 mm. Although thicknesses of 0.8 or 1.2 to 1.5 mm can also be envisaged, the thickness is not limited in respect of its upper limit. This material can for instance be supplied and processed in standard sizes, such as on a roll of 2.5 metres×40 metres, whereby solar collectors of very large size are possible such as 2.5 metres×8 metres, as well as smaller sizes in an order of magnitude of for instance 1×1 metre.

[0044] This embodiment can be provided with a black coating such as for instance soot or blackboard paint.

[0045] The throughflow rate area of the liquid for heating is preferably about 7 litres per square metre of collector surface area. This can however be adjusted depending on the desired energy absorption and liquid properties. It has been found that an ideal distance for a rib or welded joint spacing is 15 to 20 cm.

[0046] The present invention is not limited to the above described preferred embodiments; the rights sought are defined by the following claims. 

1. Solar collector, comprising: a first plate for collecting solar energy, a second plate wherein there is an intermediate space between the two plates, and a quantity of liquid which is present between the plates for absorbing the energy collected by the first plate.
 2. Solar collector as claimed in claim 1, which further comprises a framework for fixing the plates at a mutual distance along the edges thereof so as to seal the space between the plates and to create space between the plates which comprises the liquid, wherein the plates are substantially flat.
 3. Solar collector as claimed in claim 1 or 2, further comprising ribs placed between the first and the second plate in a manner such that, as seen from bottom to top, they substantially seal the space between the two plates.
 4. Solar collector as claimed in claim 3, wherein the ribs are arranged substantially parallel on two parts of the framework placed opposite one another in a manner such that the ribs form together with the framework a zigzag-shaped throughflow channel for the liquid between the first and the second plate.
 5. Solar collector as claimed in claim 1, wherein the first and the second plate are welded together along the edges with edge joints.
 6. Solar collector as claimed in claim 1 or 5, further provided with welded joints parallel to two parallel edges which extend from a side wall to a predetermined point situated at a distance from the opposite side as seen from the start of the welded joint.
 7. Solar collector as claimed in claim 1, 5 or 6, further provided with weld points in a predetermined pattern for holding the plates together.
 8. Solar collector as claimed in claim 7, wherein the plates move apart between the edge joints, welded joints and weld points.
 9. Solar collector as claimed in one or more of the foregoing claims, wherein the solar collector comprises inlet and outlet means to allow liquids to flow in and out.
 10. Solar collector as claimed in one or more of the foregoing claims, wherein the first plate is made from a non-transparent material with high heat-conducting properties.
 11. Solar collector as claimed in one or more of the foregoing claims, wherein the first plate on the solar energy-absorbing side is provided with means which improve the absorbing properties of the plate.
 12. Solar collector as claimed in one or more of the foregoing claims, wherein the means for improving the absorbing properties is a coating which comprises colouring agents.
 13. Solar collector as claimed in claim 12, wherein the coating also brigs about a low emission coefficient of the first plate so as to limit the emission of energy.
 14. Solar collector as claimed in one or more of the foregoing claims, wherein the plate material of the first plate is stainless steel.
 15. Solar collector as claimed in one or more of the foregoing claims, wherein a cover plate is mounted above the first plate so as to limit heat losses, for instance through convection.
 16. Solar collector as claimed in claim 15, wherein the cover plate is provided on the top side with means for limiting reflection.
 17. Solar collector as claimed in claim 15 or 16, wherein the cover plate is provided on the underside with means for preventing energy flows through this cover plate.
 18. Solar collector as claimed in one or more of the foregoing claims, wherein the solar collector and the cover plate are contained in a housing.
 19. Solar collector as claimed in one or more of the foregoing claims, wherein the second plate consists of material with a low coefficient of heat conduction.
 20. Solar collector as claimed in claim 18, wherein the housing is provided on the inside with insulating means for reducing heat flows on the sides and underside of the device.
 21. Solar collector as claimed in one or more of the foregoing claims, wherein during use it is disposed at the most perpendicular possible angle relative to the solar radiation.
 22. Solar collector as claimed in one or more of the foregoing claims, wherein it is disposed rotatably along one or more axes.
 23. Method for directing a solar collector of one or more of the foregoing claims so that it acquires the most perpendicular possible positioning relative to the solar radiation, comprising the following steps of: determining the direction of the sun, calculating the hereby desired position of the device, displacing the device so that this position is assumed.
 24. Method for manufacturing a solar collector as claimed in one or more of the claims 5-8, comprising the following steps of: placing two plates of substantially equal size onto one another; welding the two plates together along the edges; welding the welded joints to form the throughflow channels; welding the weld points.
 25. Method as claimed in claim 24, wherein the welding takes place with a laser welding process. 